Helium Droplets: Unique Nanoreactors for the Investigation of Molecular Dopants

Both pulsed and continuous sources of helium nanodroplets were employed for the investigation of molecular dopants. The doped helium droplets were investigated with the use of a range of techniques including electron impact ionization mass spectrometry, electronic spectroscopy and infrared (vibrational) spectroscopy. Electron impact mass spectrometry was used to investigate the influence of droplet size ( = 4000 - 80 000 helium atoms) and dopant species on the formation of helium cluster cations. The abundance of larger helium cluster cations, produced from pure helium droplets, was found to increase with droplet size until an asymptotic limit was reached for = 50 000 helium atoms. The effect of a dopant species was shown to alter the He[subscript n][superscript +]/ He[subscript 2][superscript +] (n ≥ 3) signal ratio for smaller droplet sizes and was attributed to the potential energy gradient created by the cation-dopant interaction, and its potential to draw the positive charge towards the centre of the droplet. Core-shell particles, consisting of a water core and a co-dopant outer shell, were produced using a sequential pickup technique and were analysed with electron impact ionization. Of the co-dopants used O[subscript 2], N[subscript 2], C[subscript 6]D[subscript 6] and CO[subscript 2] were found to provide a softening effect on the ionization of the water clusters, whilst CO and NO increased the fragmentation of some water cluster sizes. Results from ab initio calculations of [X(H[subscript 2]O)[subscript 2]][superscript +] cluster ions, where X = CO, N[subscript 2], Ar and CO[subscript 2], support the experimental results. A new method for recording electronic spectra of species that reside in long-lived metastable states inside the helium droplets was demonstrated using the electronic excitation of toluene into its S[subscript 1] state as an example. In another spectroscopic study, infrared depletion spectroscopy was used to record vibrational spectra of water-methane clusters. From a comparison of predicted vibrational frequencies from ab initio calculations with the experimental spectra, possible structures for the CH[subscript 4](H[subscript 2]O)[subscript n], for n = 1-3, were identified.